1. Field of the Invention
The present invention relates to a nitride-based semiconductor laser device, and more particularly, it relates to a nitride-based semiconductor laser device having nitride-based semiconductor layers formed on a substrate.
2. Description of the Background Art
A nitride-based semiconductor laser device has recently been expected for application to a light source for an advanced large capacity optical disk, and actively developed. A method of growing nitride-based semiconductor layers on a sapphire substrate thereby forming a nitride-based semiconductor laser device is known in general. In relation to this, known is a technique of forming a low-temperature buffer layer between the sapphire substrate and the nitride-based semiconductor layers in order to relax lattice mismatching between the sapphire substrate and the nitride-based semiconductor layers. This technique is disclosed in H. Amano et al., Appl. Phys. Lett. 48,353 (1986), for example. Also generally known is a technique of providing a light guide layer between an active layer and a cladding layer thereby reinforcing confinement of vertical light. This is disclosed in Japanese Patent Laying-Open No. 10-294529 (1998), for example.
FIG. 18 is a sectional view showing an exemplary conventional nitride-based semiconductor laser device, and FIG. 19 is a detailed sectional view of an emission layer of the conventional nitride-based semiconductor laser device shown in FIG. 18. The structure of this conventional nitride-based semiconductor laser device is now described with reference to FIGS. 18 and 19.
In this conventional nitride-based semiconductor laser device, a low-temperature buffer layer 102 of undoped GaN having a thickness of about 20 nm is formed on a sapphire substrate 101, as shown in FIG. 18. An n-type contact layer 103 of n-type GaN doped with Si having a thickness of about 4 μm is formed on the low-temperature buffer layer 102. The n-type contact layer 103 is partially removed to have an exposed surface and a projecting portion. An n-type anti-cracking layer 104 of n-type In0.1Ga0.9N doped with Si having a thickness of about 50 nm is formed on the projecting portion of the n-type contact layer 103. An n-type cladding layer 105 of n-type Al0.3Ga0.7N doped with Si having a thickness of about 0.5 μm is formed on the n-type anti-cracking layer 104. An emission layer 106 is formed on the n-type cladding layer 105.
As shown in FIG. 19, the emission layer 106 is constituted of an n-type light guide layer 161, an MQW (multiple quantum well) active layer 162 formed on the n-type light guide layer 161, a p-type cap layer 163 formed on the MQW active layer 162 and a p-type light guide layer 164 formed on the p-type cap layer 163. The n-type light guide layer 161 consists of n-type GaN doped with Si and has a thickness of about 100 nm. The MQW active layer 162 is formed by alternately stacking four barrier layers 162a of undoped In0.01Ga0.99N each having a thickness of about 20 nm and three well layers 162b of In0.2Ga0.8N doped with Si each having a thickness of about 2.5 nm. The p-type cap layer 163 consists of p-type Al0.2Ga0.8N doped with Mg and has a thickness of about 10 nm. This p-type cap layer 163 has a function of preventing the MQW active layer 162 from deterioration of crystals by preventing desorption of In contained in the MQW active layer 162. The p-type light guide layer 164 consists of p-type GaN doped with Mg and has a thickness of about 100 nm.
As shown in FIG. 18, a p-type cladding layer 107 of p-type Al0.3Ga0.7N doped with Mg having a thickness of about 0.5 μm is formed on the emission layer 106 (the p-type light guide layer 164). The p-type cladding layer 107 is partially removed to have a projecting portion. A p-type contact layer 108 of p-type GaN doped with Mg having a thickness of about 0.5 μm is formed on the projecting portion of the p-type cladding layer 107. The p-type contact layer 108 and the projecting portion of the p-type cladding layer 107 constitute a ridge potion 109.
A current blocking layer 110 of SiO2 having a thickness of about 0.2 μm is formed on a partial region of the exposed surface of the n-type contact layer 103, the side surfaces of the n-type anti-cracking layer 104, the n-type cladding layer 105, the emission layer 106 and the p-type contact layer 108 and the surface of the p-type cladding layer 107. A p-side ohmic electrode 111 is formed on the p-type contact layer 108. A p-side pad electrode 112 is formed to cover the surface of the p-side ohmic electrode 111 and a partial region of the surface of the current blocking layer 110. An n-side ohmic electrode 113 is formed on another partial region of the exposed surface of the n-type contact layer 103. An n-side pad electrode 114 is formed on the upper surface of the n-side ohmic electrode 113.
In the conventional nitride-based semiconductor laser device shown in FIG. 18, a voltage is forwardly applied between the p-side pad electrode 112 and the n-side pad electrode 114 so that a current flows from the p-side pad electrode 112 to the n-side pad electrode 114 through the p-side ohmic electrode 111, the nitride-based semiconductor layers 108 to 103 and the n-side ohmic electrode 113. Thus, the emission layer 106 generates a laser beam. In this case, light in the emission layer 106 is vertically confined due to difference between the refractive indices of the MQW active layer 162 and the n- and p-type cladding layers 105 and 107.
The n- and p-type light guide layers 161 and 164 having intermediate refractive indices between those of the MQW active layer 162 and the n- and p-type cladding layers 105 and 107 are provided between the MQW active layer 162 and the n- and p-type cladding layers 105 and 107 respectively so that confinement of the vertical light can be reinforced, whereby high-density light can be confined in the emission layer 106.
In another exemplary conventional nitride-based semiconductor laser device, nitride-based semiconductor layers are formed on a substrate of n-type SiC, as disclosed in Japanese Patent Laying-Open No. 11-340580 (1999), for example. In still another exemplary conventional nitride-based semiconductor laser device, nitride-based semiconductor layers are formed on a substrate of GaAs or Si.
In the conventional nitride-based semiconductor laser device shown in FIG. 18, however, AlGaN employed for the n- and p-type cladding layers 105 and 107 or InGaN employed for the MQW active layer 162 has such an inconvenience that crystal quality is remarkably deteriorated if the Al composition or the In composition is increased. Thus, it is difficult to increase the difference between the refractive indices of the MQW active layer 162 and the n- and p-type cladding layers 105 and 107 by increasing the Al composition or the In composition. Also when the n- and p-type light guide layers 161 and 164 are provided between the MQW active layer 162 and the n- and p-type cladding layers 105 and 107, therefore, optical confinement is inconveniently insufficient.
As hereinabove described, optical confinement is so insufficient in the conventional nitride-based semiconductor laser device that light tends to effuse from the emission layer 106 including the MQW active layer 162 toward the n- and p-type cladding layers 105 and 107. In general, it is conceivable that light effusing from the emission layer 106 partially propagates to the transparent sapphire substrate 1 to exert bad influence on the laser beam. More specifically, spatial distribution of light intensity is not excellently single mode but the laser beam itself is so destabilized that the shape or a spot position thereof varies during driving of the laser device. Consequently, it is difficult to stabilize the laser beam.
Further, a larger number of dislocations are formed in the low-temperature buffer layer 102 provided for relaxing lattice mismatching between the sapphire substrate 101 and the n-type contact layer 103 itself, to inconveniently result in light scattering or absorption in the low-temperature buffer layer 102. This also leads to difficulty in stabilization of the laser beam.
In the conventional nitride-based semiconductor laser device shown in FIG. 18, a p-type nitride-based semiconductor layer containing Al doped with Mg and Zn serving as a p-type dopant is remarkably deteriorated in crystal quality when increased in thickness, due to formation of cracks or the like. Therefore, the thickness of the p-type cladding layer 107 consisting of p-type AlGaN must be suppressed to several 100 nm, and hence it is difficult to increase the distance between the emission layer 106 and the p-side ohmic electrode 111. Thus, the distance between the emission layer 106 and the p-side ohmic electrode 111 is so small in the conventional nitride-based semiconductor laser device that the p-side ohmic electrode 111 inconveniently absorbs intense light around the emission layer 106. In this case, the emission wavelength of the nitride-based semiconductor laser beam is so smaller than that of an infrared or red semiconductor laser beam that the p-side ohmic electrode 111 inconveniently easily absorbs the nitride-based semiconductor laser beam. Consequently, a threshold current or an operating current is disadvantageously increased.
In a further conventional nitride-based semiconductor laser device employing a substrate of GaAs or Si, the substrate of GaAs or Si having a band gap sufficiently smaller than the band gap of an active layer (the quantum level of a well layer when the active layer has an MQW structure) can absorb light effusing from an emission layer dissimilarly to a sapphire substrate. However, the band gap of GaAs or Si is so small as compared with that of the emission wavelength that the substrate of GaAs or Si inconveniently excessively absorbs light. Consequently, the threshold current or the operating current is disadvantageously increased.
In the aforementioned conventional nitride-based semiconductor laser device employing a substrate of SiC disclosed in Japanese Patent Laying-Open No. 11-340580, it is difficult for the substrate of SiC, i.e., an indirect transition semiconductor hardly absorbing light, to effectively absorb light effusing from the emission layer due to a band gap equivalent to that of the active layer consisting of a nitride-based semiconductor. Consequently, the laser beam is disadvantageously destabilized.
A p-type nitride-based semiconductor doped with Mg or Zn has an impurity level deeper than that of an n-type nitride-based semiconductor doped with an n-type dopant, and hence a p-type nitride-based semiconductor layer remarkably absorbs light. When p-type nitride-based semiconductor layers are formed on a substrate, therefore, it is disadvantageously difficult for light effusing from the emission layer to efficiently effuse into the substrate.